Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates generally to apparatus and method for power amplifying radio frequency (rf) or microwave rf signals. More particularly, the present invention pertains to an rf power amplifier in which the rf output is selectively phase shifted at angles up to, and beyond, 270 degrees in response to a variable phase-control voltage, or is binary-phase-shift-key modulated in response to digital inputs of zero and one.
2. Description of Related Art
Binary-phase-shift-key (BPSK) modulation is a form of digital modulation in which the rf carrier is phase shifted 180 degrees (inverted) as a digital input changes from 0 to 1. A demodulator, that is a part of an rf receiver, demodulates these phase inversions to recover the original digital stream. Commonly, demodulation is accomplished by a Costas Loop.
A common encoder consists of the rf carrier being inserted into an rf input port of a mixer while a digital input is inserted into an input port of a local oscillator. As the digital input into the input port of the local oscillator changes from an above ground voltage (1) to below ground (0), the output of the mixer changes phase from 0 degrees to 180 degrees.
If the input to the local oscillator were to change polarity (0 to 1, or 0) instantaneously, the phase of the rf output would also change polarity instantaneously. This would cause the output rf spectrum to spread to an unacceptable width.
To prevent this spread in the rf output spectrum (spectrum splatter), commonly, the input to the local oscillator port is filtered (usually with a Bessel filter. As a result, the rf output decreases as the voltage to the input port of the local oscillator is decreased, and the rf output decreases to 0 when the input to the local oscillator passes through 0.0 volts. Then the rf output increases in amplitude (with inverted phase) as the voltage to the local oscillator input increases to the opposite extreme.
Therefore, as the filtered input passes through 0 volts as the polarity changes, the rf output also passes through a 0 rf output condition. This creates a problem in that the rf power amplifier section stages of conventional transmitters consists of several stages biased to Class C. In a Class C amplifier, a 0 rf input signal causes the amplifier to shut off. If a Class C amplifier were to follow the above-described encoder, it would shut off every time the input data changes state. This turning off and on of the Class C stages would cause the rf output to occupy far more of the frequency spectrum than allowed by federal regulations.
The present invention solves the above-mentioned problems with phase-shifting in general, and binary-phase-shift-key (BPSK) modulation in particular, in that the rf output stays relatively constant as the phase shifts. In one embodiment the phase shifts up to 180 degrees generally linear with a variable phase-control voltage, or shifts 180 degrees in response to a filtered BPSK input.
More particularly, the phase shifts from 0 to 90 degrees in response to a phase-control voltage increasing from 0.0 volts dc to 5.0 volts dc during which time the rf output remains substantially constant; and the rf output continues to be relatively constant as the phase shifts from 90 degrees to 180 degrees as the filtered BPSK input increases from 5.0 volts dc to 10.0 volts dc.
Since the rf output remains substantially constant during changes in the phase angle, turning off and on of Class C stages following the encoder is avoided, frequency splatter is avoided, and the occupied frequency spectrum of the rf output follows theoretical values more closely.
The present invention utilizes solid-state amplifying devices, preferably FETs in a totem-pole arrangement. As taught by Lautzenhiser et al. in U.S. patent application Ser. No. 10/028,844, filed Dec. 20, 2001, which is incorporated herein by reference thereto, two or more solid-state devices, or FETs, can be series connected, in a totem-pole arrangement, to dividingly share a source-voltage that is too high for a single solid-state amplifying device, or FET.
In the present invention, two or more solid-state amplifying devices, or FETs, are connected in series in a totem-pole arrangement, and they dividingly share, or selectably utilize, the source-voltage. That is, to phase shift the rf output to some angles the entire source-voltage is utilized by a selected one of the solid-state amplifying devices, or FETs, and to phase shift the rf output to other phase angles the source-voltage is dividingly shared by two adjacent ones of the solid-state amplifying devices.
Therefore, source-voltage sharing in the present invention is for an entirely different purpose, and functions entirely different, than that of the aforementioned Lautzenhiser et al. patent application. However, the two inventions share a common problem. Unless proper rf decoupling is achieved, the maximum rf power output is extremely limited.
More particularly, totem-pole arrangement of solid-state amplifying devices was taught in a paper published in IEEE Transactions on Microwave Theory and Techniques, Volume 46, Number 12, of December 1998, in an article entitled, xe2x80x9cA 44-Ghz High IP3 InP-HBT Amplifier with Practical Current Reuse Biasing.xe2x80x9d As taught in the IEEE article, in totem-pole circuits two, or more, solid-state amplifying devices are used in series for dc operation, but they are used in parallel for rf operation, thereby supposedly solving the disparity between source-voltages and working voltages.
However, totem-pole, voltage-dividing, or current-sharing circuits, had been used only at low rf powers, as in the above-referenced article wherein the power was in the order of 10 milliWatts. At higher rf powers, inadequate rf decoupling has resulted in low power efficiency, oscillation, a decrease in reliability of the circuits, and destruction of the solid-state amplifying devices.
In contrast, to the extremely low rf outputs in which the prior art has been able to utilize totem-pole circuity, Lautzenhiser et al., in the aforementioned patent application, teach apparatus and method for rf decoupling in which the principles thereof may be used to make totem-pole circuits that are limited only by power limitations of the solid-state amplifying devices that are used in the totem pole.
More particularly, in totem-pole circuits, problems with rf decoupling are most severe between the solid-state amplifying devices. For instance, when using FETs, rf decoupling is the most critical with regard to a source terminal of any FET that is connected to a drain terminal of a next-lower FET. Capacitors and rf chokes are used for rf decoupling and rf isolating, but selection and design of capacitor decoupling is the most critical.
The next most critical location for rf decoupling is the source terminal of the lower FET when the source terminal of the lower FET is connected to an electrical ground through a resistor, as shown herein. However, if a negative bias voltage is used for the gate of the lower FET, and the source is connected directly to an electrical ground, this source terminal is already rf decoupled.
Other critical rf decoupling problems are those associated with the source-voltage to the drain of the upper FET and bias voltages to the gates of the FETs. The use of properly designed rf chokes are sufficient to provide adequate rf decoupling in these locations.
Unless rf decoupling is provided, as taught by Lautzenhiser et al. in the above-referenced patent application, reduced efficiency will certainly occur, and both instability and destruction of the solid-state amplifying devices are likely. This is true for both totem-pole circuitry in which a source-voltage that is excessive for a single solid-state amplifying device is dividingly shared, and for phase-shifting as taught in the present invention.
The present invention provides variable phase-shifting rf power amplifiers in which, in various embodiments taught herein, the rf output can be selectively shifted up to 90 degrees, up to 180 degrees, or up to 270 degrees in response to variable or preselected phase-control voltages. The resultant rf output is phase shifted without appreciably affecting the rf output power during phase-shifting, while remaining at any phase-shifted angle, or even when the rf output is digital-phase-shift-key (DPSK) modulated.
Each of the variable phase-shifting rf power amplifiers of the present invention includes a phase splitting/combining rf power amplifier and a phase control. The phase splitting/combining rf power amplifier includes 2, 3, or 4, solid-state amplifying devices, preferably FETs, for phase-shifting up to 90, 180, or 270 degrees, respectively.
The solid-state amplifying devices (FETs) in the phase splitting/combining rf power amplifier are controlled by 1, 2, or 3 phase-shifting voltages. That is, for 180 degree phase-shifting, three solid-state amplifying devices are required, and two phase-shifting voltages are required. The phase controls of the present invention generate the phase-shifting voltages in response to a single phase-control voltage. However, for 90 degree phase-shifting, only one phase-shifting voltage is needed, so the phase angle increases in response to, and substantially linear to, a phase-control voltage.
In the phase splitting/combining rf power amplifiers of the present invention, an rf input is phase split into 2, 3, or 4 rf signals. The phase-split rf signals are selectively amplified in response to phase-shifting voltages. Then the phase-split rf signals, that have been selectively amplified, are combined to provide an rf output signal whose power remains substantially constant during phase shifts up to 90 degrees, 180 degrees, 270 degrees, or more.
Phase splitting is accomplished by such devices as quadrature power splitters, or by a combination of 180 degree and 90 degree power splitters. Preferably, power combining is accomplished by one or more in-phase power combiners.
Preferably, the solid-state amplifying devices are connected in totem-pole arrangement with the solid-state amplifying devices selectively and dividingly sharing the source-voltage. Therefore, rf decoupling is critical and is accomplished as taught by Lautzenhiser et al. in the above-referenced patent application.
Finally, as taught by Lautzenhiser et al. in the above-referenced patent application, in designs in which the source terminal is the mounting flange of a packaged FET, as is common in high-power solid-state amplifying rf power devices, a mounting technique is used that avoids both over heating and the resultant danger of destroying the internal junctions of the solid-state amplifying device, while maintaining electrical isolation from a circuit ground.
In a first aspect of the present invention, a method for phase-shifting an rf output comprises: phase-splitting an rf input into a plurality of rf signals that are at different phase angles; amplifying a selected one of the rf signals; simultaneously amplifying an other of the rf signals; inversely controlling gains of the amplifying steps; and combining the rf signals into the rf output subsequent to the amplifying steps.
In a second aspect of the present invention, a method for phase-shifting an rf output comprises: splitting an rf input into first and second rf signals that are at different phase angles; inputting the first rf signal into a first solid-state amplifying device; inputting the second rf signal into a second solid-state amplifying device; amplifying the first and second rf signals with selective and different gains; and combining the rf signals subsequent to the amplifying step.
In a third aspect of the present invention, a method for binary-phase-shift-key modulating comprises: splitting an rf output into 0, 90, and 180 degree rf signals; separately amplifying the rf signals; combining the separately amplified rf signals into a single rf output; and preventing the single rf output from decreasing to 0 degrees when the rf output is phase-shifted 180 degrees.
In a fourth aspect of the present invention, an rf power amplifier comprises: a first solid-state amplifying device having a first higher-voltage terminal that is connected to a higher-voltage, having a first lower-voltage terminal, and having a first control-voltage terminal; an rf choke being connected to the first lower-voltage terminal; a second solid-state amplifying device having a second higher-voltage terminal that is connected to the rf choke distal from the connection thereof to the first lower-voltage terminal, having a second lower-voltage terminal that is connected to a lower voltage, and having a second control-voltage terminal; means, being connected between the first lower-voltage terminal and an electrical ground, for decoupling the first and second solid-state amplifying devices; and the means for decoupling comprises an rf effective series resistance of less than 0.4 divided by an rf output, in Watts, of the rf power amplifier.
In a fifth aspect of the present invention, a method comprises: phase-splitting an rf input into a plurality of rf signals whose phase angles encompass first and second phase angles; separately and simultaneously amplifying the rf signals; selectively proportioning gains of the amplifying steps; combining the separately and simultaneously amplified rf signals into a single rf output; and phase-shifting the rf output to selected phase angles between the first and second phase angles as a function of the selective proportioning step.